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Link hints only vary per-host, so splitting link hints into their own
optimization type will allow us to send them at host-granularity rather
than copying them for each pattern on a host. This will significantly
reduce hint size.

This change is behind a flag so we can prepare the server-side changes
in the meantime.

One-pager: https://docs.google.com/document/d/1ZUI6V7jxT0WgTTC8mTUcM91XZW8Yv6skqHFh0NuF4gk/edit

BUG=1139455

Change-Id: I24f1519ae3f1719d445c5ef4a67e571602d2306b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2533800Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Megan Jablonski <megjablon@chromium.org>
Reviewed-by: Sophie Chang <sophiechang@chromium.org>
Commit-Queue: Josh Simmons <jds@google.com>
Cr-Commit-Position: refs/heads/master@{#826954}